The field of the invention relates to idle speed control systems for internal combustion engines. In particular, the field relates to idle speed control systems for direct injection spark ignition engines.
In conventional port injected engines, which induct a mixture of air and atomized fuel into the combustion chambers, idle speed control systems are known which adjust idle speed by controlling the air throttle. It is also known to control idle speed by advancing or retarding ignition timing. An example of such a system is disclosed in U.S. Pat. No. 5,203,300.
The inventor's herein have recognized numerous problems when applying known idle speed control systems to direct injection spark ignition engines in which the combustion chambers contain stratified layers of different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. Use of conventional idle speed control systems for this type of engine is recognized by the inventors herein to be inadequate because stratified operation is unthrottled so the throttle is not a viable control variable. And ignition timing is not a viable control variable because the timing must be slaved to the time a rich air/fuel strata is formed near the spark plug. These problems are further exasperated in direct injection spark ignition engines which have two modes of operation--the stratified mode discussed above and a homogeneous mode in which a homogeneous air/fuel mixture is formed at the time of spark ignition.